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Fig. ^^'2

Fig. 113.

I'^iG. 111. A photogniph frnni an uiukt tliird iimLir split iiiesio-distally in llit' axial plane,
showing a whitened area of beginning decay in a pit in the occlusal surface, and a slighter decay
of enamel in the mesial surface, showing as a whitened uvra.
Fig. 112. A diagrammatic representation of the penetration of enamel and dentin in the central
pit of the occlusal surface, and also on the smooth buccal surface, in a molar tooth cut bucco-
lingiially in the axial plane. In each, the particular direction of the penetration in each tissue is
shown by small arrows. In the occlusal pit there is no spreading of decay on the surface of the
enamel ; the area of enamel involved is broadest at the dento-enamel junction, forming a conical area
of (Jeca^- with the base at the dento-enamel junction ;;nd the apex at the surface of the enamel. On
the smooth enamel of the buccal surface the decay spreads laterally on the surface, each part of
the widened area of beginning penetrating in the line of the length of the enamel rods, which
gives a conical area of decay with the base of the cone at the sv:rface of the enamel. Notice that
after the penetration of the enamel, decay at once spreads laterally along the dento-enamel junction
in every direction. Tliis direction of spreading is confined to an area very close to the dento-
enamel junction, but as each new dentinal tubule is reached, spreading along it toward the pulp
of the tooth occurs. In the first ones entered it goes the deepest because they begin first, and this
gives a conical area of decay.
Fig. 113. A photograph from a split upper molar, showing a blackened area of decay in the
dentin, which has bcgim by the penetration of the enanul through the occlusal pit.
16a